The present invention relates to improvements in fluid proportioning valves such as disclosed in U.S. Pat. No. 3,519,018 issued to Cole et al on July 7, 1970 and U.S. Pat. No. 3,646,966 issued to E. Allen Smart on Mar. 7, 1972. In valves of this type, a valve body is provided with an internal, generally cylindrical recess having a pair of diametrically opposed inlet ports in the side wall. A sleeve is inserted in the recess which has a flexible sealing cushion in front of each inlet port to function as an opening and closing valve, and to control the valving movement of these cushions a control rod cam is inserted within the sleeve. The control rod cam has a cylindrical portion that can be aligned with the inlet ports to press the cushions tightly against the inlet ports to close them, and it also has a tapered portion extending from the cylindrical portion that can be aligned with the inlet ports to control fluid flow through the ports. A lobe along one side of the tapered portion that functions to enhance the port closing action of the control rod characterizes the structure of the Smart patent, and the purpose of the lobe was to overcome problems of looseness which were encountered at times during mass manufacture of mixing valves. In particular, that patent intended to hold one valve tightly shut while the other was set for maximum fluid flow. The prior art control rod cams have been generally satisfactory and provide a significantly successful device in the art. However, such control rod cams do not impart fully desired flow conditions for a mixing valve, and do not exhibit such a high degree of sensitivity of adjustment as is now desired. To alleviate this drawback, the present invention has been proposed.